1. Field of the Invention
The present invention relates generally to the measurement of surface roughness and pertains particularly to an instrument for non-contact in process surface roughness measurement.
2. Discussion of Prior Art
Surface smoothness is an important consideration in the manufacture of many products. The manufacturing of sheet metal is one industry where surface smoothness must be maintained at a high standard. Sheet metal, such as sheet steel and aluminum are manufactured by squeezing the metal between opposed high-pressure rollers and frequently at high temperatures. The high-pressure rollers become rough in the process of forming the sheet metal, thereby transferring their roughness to the sheet metal. The rollers must be removed from the machinery and resurfaced by regrinding to a smooth surface to continue the rolling operation. Rollers for the aluminum industry are typically removed and resurfaced on a daily basis. Rollers for sheet steel deteriorate rapidly and are typically removed and resurfaced in a matter of a few hours.
The process of checking the smoothness of the rollers and the sheet metal during the process is critical to the manufacturing process. The conventional approach to surface smoothness monitoring is by the use of a mechanical stylus profilometer. In order to check for surface smoothness the production line must be stopped. The parts to be tested are moved to a laboratory for the testing to take place, or a small portable profilometer is mounted on the static part to perform a measurement. This disrupts the production line and is time consuming.
A stylus profilometer utilizes a fine stylus tip, usually a few microns in width, which is brought into contact with the surface and is dragged across the surface for a distance of typically one to two mm. As it is dragged across the surface, the stylus moves up and down as it encounters scratches, pits, and general roughness on the surface. This vertical motion is carefully monitored and recorded and then used to calculate surface roughness. The motion is similar to a phonograph needle, except on a finer scale.
A number of drawbacks to these instruments and their use is lack of sensitivity, accuracy, and repeatability, particularly on higher quality surfaces. They are very sensitive to vibrations and often leave visible scratches on surfaces that they measure, either damaging the part or, at the very least, leaving one to wonder what they are really measuring. This measuring process is very slow and covers an exceptionally small surface area on a surface. This makes high volume, in-process and large area testing virtually impossible.
Attempts to overcome the drawbacks of these instruments were proposed in the 1970's by the introduction of optical profilometers. These were used primarily in the precision military and aerospace optics industry. Stylus profilometers were incapable of measuring state of the art surfaces achievable by these users. Optical profilometers are leading edge, complex and difficult to operate. Several different versions of these systems have been developed and are presently in use.
These instruments involve the coupling of a laser interferometer to a precision optical microscope, wherein a laser beam enters the microscope and then is split in half inside the microscope. Half of the beam is focused by the microscope onto a sample to be measured and the other half is focused onto a known reference surface. The laser beam bounces off these two surfaces inside the microscope. Optical interference patterns are created by the surface finish on the test piece and are measured by the instrument using this information to calculate surface roughness. These instruments also have a number of drawbacks, including extremely high cost, too slow for manufacturing quality control or process control application, and require flat surfaces to measure. In addition, they can only operate in a controlled laboratory or a highly controlled and clean manufacturing environment.
Atomic force microscopes (AFM's) and scanning tunneling microscopes (STM's) are high technology surface roughness measuring tools which were introduced in the 1980's. Their principle of operation involves an absolute in precision mechanical device motion control and vibration isolation. An ultra-fine stylus tip like a phonograph needle, only a few molecules in diameter, is brought to within atomic distance from a surface and an atomic tunneling current is created between the molecules in the stylus tip and the surface to be measured (the surface must be electrically conductive). The instrument then precisely maintains the atomic force and motions the stylus a few microns (1/100th the width of a human hair) across the surface. Roughness, defects, or scratches cause the stylus to move up and down during the scan in order to maintain the "atomic force". The up and down motion is monitored and measured very precisely and this information used to determine roughness or finish over the measured area. The instrument is enormously sensitive and can resolve individual molecules and atoms under favorable conditions. The major drawbacks of this approach are that the instruments can only be used in laboratory environments due to their extreme sensitivity to vibration and harsh environments, and they are extremely expensive.
Attempts have also been made in recent years to develop an optical non-contact sensor of surface roughness which involved the principal of illuminating a surface with either an incoherent or coherent laser source and monitoring the specular light being reflected off the surface. As the surface roughness changes, the relative intensity, polarization, or direction of the specular light changes. Examples of these attempts are disclosed in U.S. Pat. Nos. 5,162,660 issued to Popil; 4,511,800 and 4,803,374 issued to Monfort et al; and 4,973,164 issued to Weber et al.
Attempts to measure and evaluate overall integrated scatter intensity are disclosed, for example, in U.S. Pat. Nos. 4,360,275 issued to Louderback, and 4,972,092 issued to Schmitt et al. Attempts to integrate the overall intensity and angular distribution of light reflected and scattered from the surface for analysis is addressed in U.S. Pat. Nos. 5,164,790 to McNeil et al and 4,334,780 to Pernick.
These represent an attempt to design instruments to quantify scatter for analysis and use in aerospace and astronomical optical system designs. Most of these earlier laboratory models were designed for small samples to be mounted in a holder with some laser source illuminating this sample and detector on a motorized stage scanning in an arc around the sample to record the scatter angle and intensity information. A mathematical technique known as fourier transform analysis converts the density of laser scatter at specific angles to the roughness value of the measured surface. The instruments have been complex R&D machines with selling prices above $100,000.00. Few of these instruments are in use and are typically considered solely as instruments for measuring scattered light from very smooth optical surfaces. The measurement of very smooth optical surfaces with scatter and conversion of these scatter signals to roughness values is fairly well understood and the technique has evolved as a complimentary and competitive technique to optical profilometry.
Attempts to adapt this technology to high volume surface measurement for general manufacturing has not been successful. The attempts have been made involving some form of laser illumination with one or more discrete detectors positioned at various scatter angles to sample a portion of the scatter field. Attempts to convert these limited measurements to surface roughness values involve making assumptions about the rest of the scatter patterns that these systems do not measure. These assumptions are often substantially incorrect, resulting in significant inaccuracies in the roughness determination.
Another drawback to most of the prior instruments is that the sensors are positioned so that the receiving phase is normal to the reflected rays from the measured surface. This results in any light not being absorbed by the receiver being reflected back to the measured surface, and reflected again and re-received. In order to solve this problem, these instruments have had elaborate and expensive black, beam folding and beam absorbing traps or dumps built into the detection system.
Accordingly, there is an evident need for an instrument that has the capability of reasonably accurate, rapid, non-contact, non-destructive, in-process measurement of surfaces as they are being created.